Update CLI installer to auto-upgrade

When you run the `az bicep install` command, if Bicep is already installed, it doesn't take any action. This includes when an older version of Bicep is installed and a newer version is available.

This means that for scripts, and for documentation instructions, we need to have instructions along the lines of "Run `az bicep install` and then `az bicep upgrade`" to ensure the user has the latest version.

It would be nice if we could issue a single command that would both install the latest Bicep version, and upgrade an existing version.